These days, access networks, also called passive optical networks (PON), are used, for connecting a plurality of customers to a core network of data transportation.
In such an access network, the interconnection between the core network and the access network is given at a so-called central office, which contains an optical line terminal (OLT).
The OLT is connected via at least one optical fiber, preferably called optical feeder fiber, to a so-called remote node. At this remote node, an optical downlink signal transmitted by the OLT is split onto different optical branches, to which one or more customers are connected by means of optical network units (ONU).
The customers send via the optical branches optical uplink signals towards the remote node, which combines these optical uplink signals to a combined uplink signal and transmits this via the optical feeder fiber to the optical line terminal.
In order to determine the transmission properties of the transmission channel into which a device transmits an optical signal, a measurement technique of optical time domain reflectometry (OTDR) can be applied. In such an OTDR measurement, a reflection profile of the transmission channel is estimated. Preferably, the technique of OTDR is carried out at the OLT.
For the purpose of OTDR, a measurement signal in the form of an optical pulse may be transmitted into the optical channel. The received response signal, which is reflected by the optical channel, can then be traced in the time domain, for determining the reflection profile.
In a more advanced technique of OTDR, a measurement signal may be generated as an optical signal carrying a correlation sequence. The received response signal is then recorded and used for determining the reflection profile. The reflection profile can be determined, by correlating a sampled version of the received response signal with the initial correlation sequence itself. In the case, that the auto-correlation function of the correlation sequence is equal to or approximated by the dirac delta function, the result of the correlation yields an estimate of the impulse response of the optical channel in the time domain, which is an approximation of the reflection profile.
When transmitting an optical transmission signal carrying transmission data into the optical channel using a transmission device, it is one possibility to carry out the technique of OTDR by using a separate device. The transmission device and the separate OTDR device are in this case both coupled to the same optical channel, preferably via an optical coupler. The optical channel is preferably an optical fiber connected to the devices.
A more advanced technique is that of embedded OTDR, in which the transmission device itself contains the hardware for generating the optical transmission signal as well as the hardware that is necessary for carrying out an OTDR measurement. Preferably, the optical transmission signal is directly modulated in dependence on a correlation sequence, wherein the frequency of this direct modulation is chosen such that it does not disturb data reception at a receiving side. After transmitting the optical transmission signal, carrying the directly modulated measurement signal, into the optical channel, the response signal of the optical channel can be obtained, by filtering out from received optical signal that frequency, at which the optical transmission signal was modulated.
This received response signal can then be used for determining a reflection profile via the technique of signal correlation as it has been described previously above.